Immunoassays utilizing an antigen-antibody reaction can detect a component or a substance present in a specimen or a sample with high sensitivity. Thus, in the field of clinical tests, such immunoassays have been used for testing various kinds of specimens and the like, such as blood (plasma, serum, whole blood), urine, cerebrospinal fluid, and feces. Examples of the immunoassay utilizing an antigen-antibody reaction include various kinds of assays, such as an enzyme immunoassay (EIA), a fluorescence immunoassay (FIA), a chemiluminescent immunoassay (CLIA), an immunochromatography assay, a turbidimetric immunoassay (TIM, and a latex turbidimetric immunoassay (LTIA).
Among these, the turbidimetric immunoassay, the latex turbidimetric immunoassay, and a latex agglutination assay that is performed on a slide glass (hereinafter, these three assays may be referred to generically as “immunoagglutination assays”) are called homogeneous immunoassays, because they operate without performing B/F (Bound/Free) separation for separating antigens and unreacted antibodies. The immunoagglutination assays have been applied to a large number of items in clinical tests, e.g., CRP (C-reactive protein), ASO (antistreptolysin O), RF (rheumatoid factors), microalbumin in urine, and elastase, because they are excellent in simplicity and rapidity of operation.
Specimens to be subjected to these immunoagglutination assays generally are blood (serum, plasma, whole blood), urine, cervical mucus, etc. On the other hand, items to be measured by testing specimens from nasal secretion, collected as nasal swab, nasal aspirate, nasal wash, etc., include those used for determining whether the patient is infected with respiratory infectious diseases, such as influenza viruses and RS viruses. However, these items are all measured by an immunochromatography assay or an EIA using a membrane filter. A measurement method that operates using an immunoagglutination assay and is applicable to specimens from nasal secretion has not yet been in practical use (see Non-Patent Documents 1 to 4, for example).
Many specimens from nasal secretion have some degree of viscosity, although the viscosity varies from one specimen to another. As can be seen from this fact, the specimens from nasal secretion contain, in addition to a target analyte, a very high content of macromolecular substances such as glycoproteins. It has been known that, in the immunochromatography assay or the EIA using a membrane filter, nonspecific reactions may be caused by any of the substances that are present with the analyte in the specimen to bring about a false test result, although the substance causing the nonspecific reactions is not yet identified. When the test result incorrectly shows false positive, it causes a delay in identifying the actual cause of the disease. Moreover, it may lead to an inappropriate treatment that may aggravate the symptoms of the disease. On this account, the specimens from nasal secretion are subjected to measurement after lowering the viscosity thereof by adding a detergent or after removing solid components (the substances present with the analyte) contained therein with a filter paper or a filter, for example. However, still, a large number of false positive results are caused by the nonspecific reactions.
The inventors of the present invention conducted a keen study in order to realize the measurement on a specimen from nasal secretion by a simple immunoagglutination assay, as had already been realized by the immunochromatography assay or the EIA using a membrane filter. The inventors found out that, when the specimen was pretreated merely by adding a detergent or by removing solid components beforehand, a large number of non-specific reactions still were caused so that the specimen that was supposed to give a negative result gave a positive result, i.e., a false positive result.    Non-Patent Document 1: The Journal of the Japan Pediatric Society, Vol. 108, No. 3, pp 406-411 (2004)    Non-Patent Document 2: The Journal of the Japanese Association for Infectious Diseases, Vol. 78, No. 9, pp 865-871 (2004)    Non-Patent Document 3: The Journal of the Japanese Association for Infectious Diseases, Vol. 77, No. 12, pp 1007-1014 (2003)    Non-Patent Document 4: The Japanese Journal of Medical Technology, Vol. 52, No. 2, pp 141-144 (2003)